As is known in the art, switched-mode power supply circuits incorporate a switching regulator to convert electrical power efficiently. The switched-mode power supply circuits transfer power from a source to a load while converting voltage and current applied to the input of the circuit to an output voltage and current suitable for the load. The switched-mode power supply circuits consist of a power stage and a control circuit. The power stage performs the basic power conversion from the input voltage to the output voltage and includes switches and the output filter. The control stage receives necessary feedback signals from the power stage and control signals from system operating functions. The feedback and control signals are interpreted to provide the driving signals for the power stage.
In current hand-held mobile electronic devices such as cellular telephones, tablet computers, portable media players and the like require a higher dynamic range of output current from the switched-mode power supply circuits. What is needed is that as the range of output currents requirements expands, the switched-mode power supply circuits must operate more efficiently over a broad range of output currents.
Multi-phase switched-mode power supply circuits include a quantity of switched-mode converter circuits that are coupled in parallel to deliver high output currents to a load. The multiple parallel switched-mode converter circuits provide an energy efficient DC/DC converter for supplying high output currents. The corresponding switching transistors in each switching stage may be switched so that input current flows into only one or more regulator stage at a time. The switching stages switch sequentially to avoid simultaneous switching and to smooth the input/output current to reduce the amplitude of the output ripple current, input ripple current, and output ripple voltage.
Switching loss and DC loss degrades the efficiency of a switched-mode converter circuit. The DC loss is due to the voltage drop across resistances such as on-resistance of the switching devices in the power stage and it is proportional to the square of the load current. Contribution to the efficiency is proportional to the load current and dominant for higher load current. To improve the efficiency for higher load current, activating multiple phases in parallel reduces effective on-resistance. However, switching loss of the switching devices in the power stage is almost constant regardless of the load current. For lower load current, the switching loss contribution becomes dominant, as the DC loss is essentially negligible. At the lower load currents, the number of active phases should be minimized for reducing the switching loss and improving the efficiency.
FIG. 1 is a plot of efficiency of a single phase switched-mode converter circuit and a four phase switched-mode converter circuit versus output current of the prior art. The output current 5 of the single phase switched-mode converter circuit is more efficient than the output current 10 of the four phase switched-mode converter circuit until the output current level reaches the crossover current level t0 that in this example is approximately 1.6 A. Once the load current is greater than the crossover current level t0, the four phase switched-mode converter circuit is efficient.